https://doi.org/10.1177/0963721419857759 Current Directions in Psychological Science 2019, Vol. 28(4) 380–386 © The Author(s) 2019 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0963721419857759 www.psychologicalscience.org/CDPS ASSOCIATION FOR PSYCHOLOGICAL SCIENCE An unsolved mystery eludes social- and cognitive- neuroscience research: Why do the same brain regions that support social cognition also spontaneously engage during rest? On the one hand, a medial frontoparietal network has been consistently associated with thinking about ourselves and others (Denny, Kober, Wager, & Ochsner, 2012; Frith & Frith, 2006; Saxe & Kanwisher, 2003; Van Overwalle & Baetens, 2009). On the other hand, these same regions are a part of the default net- work, so named because they increase engagement when participants take mental breaks (i.e., rest; Raichle et al., 2001; Shulman et al., 1997). The anatomical simi- larity between the social brain and the default brain is well documented: Several review articles and meta- analyses have highlighted the overlap (e.g., Andrews-Hanna, Saxe, & Yarkoni, 2014; Buckner, Andrews-Hanna, & Schacter, 2008; Mitchell, 2006; Schilbach, Eickhoff, Rotarska-Jagiela, Fink, & Vogeley, 2008; Spreng, Mar, & Kim, 2009). Yet why this overlap occurs is poorly under- stood. Does the tendency to engage these regions by default during rest have particular social functions, and if so, what might these be? Here, it is proposed that the default network performs at least two critical social functions during rest: social priming and social consoli- dation. The Social Brain Social cognition refers to how people make sense of other people as well as themselves (Fiske & Taylor, 2008), and such processes are reliably associated with a specific network in the brain. The medial frontopari- etal network, including medial prefrontal cortex (MPFC), precuneus/posterior cingulate (PC/PCC), and temporo- parietal junction (TPJ; Fig. 1a), corresponds with high- level social cognition, such as mental-state and personality-trait attribution (Mitchell, Macrae, & Banaji, 2004; Saxe & Kanwisher, 2003; Spunt, Satpute, & Lieberman, 2011; Van Overwalle, 2009) and social-hierarchy and stereotype representation (Contreras, Banaji, & Mitchell, 2012; Parkinson, Kleinbaum, & Wheatley, 2017; Zerubavel, Bearman, Weber, & Ochsner, 2015). These findings are consistently replicated, and they also map onto real-world social outcomes. Individuals with an autism spectrum disorder, who struggle to understand 857759CDP XX X 10.1177/0963721419857759MeyerSocial Cognition and the Default Network research-article 2019 Corresponding Author: Meghan L. Meyer, Dartmouth College, Department of Psychological and Brain Sciences, HB 6207, Hanover, NH 03755 E-mail: meghan.l.meyer@dartmouth.edu Social by Default: Characterizing the Social Functions of the Resting Brain Meghan L. Meyer Department of Psychological and Brain Sciences, Dartmouth College Abstract Social-neuroscience research has identified a set of medial frontoparietal brain regions that reliably engage during social cognition. At the same time, cognitive-neuroscience research has shown that these regions comprise part of the default network, so named because they reliably activate during mental breaks by default. Although the anatomical similarity between the social brain and the default brain is well documented, why this overlap exists remains a mystery. Does the tendency to engage these regions by default during rest have particular social functions, and if so, what might these be? Here, it is suggested that the default network performs two critical social functions during rest: social priming and social consolidation. These constructs will be defined, recently published empirical findings that support them will be reviewed, and directions for future research on the topic will be proposed. Keywords default network, memory, rest, resting state, social cognition